The present invention relates to an improved intrusion warning system of the type wherein the presence of an intruder within a given area is detected by determining the changes in the level of a quasi-stationary electric field caused by the movement of an intruder within the electric field. More particularly, the present invention relates to an improvement of the intrusion warning system disclosed in the above-identified U.S. patent application Ser. No. 612,918, the subject matter of which is incorporated herein in its entirety by reference.
According to the system disclosed in the above-identified application, a quasi-stationary electric field is produced by means of a field wire which is insulated from ground and which is connected by a voltage amplifier to the output of a frequency controlled oscillator which produces an output signal having a wavelength which is very long compared to the length of the field wire and a frequency which is in the range of from 1 to 40 KHz. The electric field is detected by means of an electrode or a receiving antenna which is disposed within the electric field and which is connected by an amplifier to an AM detector. The output signal from the detector is fed to a high gain voltage amplifier and filtering arrangement which provides output signals only within the relatively low frequency range of 0.2 to 2 Hz associated with movement of an intruder. The output signal from this amplifying and filtering arrangement is then fed to a threshold circuit which produces an output signal whenever the input signal thereto exceeds a predetermined threshold value for a predetermined period of time, and this output signal is then fed to an alarm producing circuit. Thus, an alarm is only produced if the sensed change in the electric field amplitude exceeds a predetermined or preset level, the rate of change is between 0.2 Hz and 2.0 Hz, and the electric field change persists for a predetermined or preset length of time.
As is further disclosed in the above identified application, the intrusion warning system may be utilized to provide protection for the perimeter, or at least a portion of the perimeter, of a given area. In such case, the field wire comprises a length of wire which extends along the portion of the perimeter to be protected and is mounted on and suspended between a plurality of fence posts distributed about the perimeter by means of respective insulators. The receiving antenna, in this case, likewise comprises a length of wire which is similarly mounted on the fence posts and extends along the perimeter of the area to be protected substantially parallel to the field wire. In such systems, the length of the field wire may be up to approximately 1000 feet. With such perimeter type insulations, it has been found that under certain environmental conditions, the system may produce alarms in the absence of any movement by an intruder in the produced electric field, i.e., false alarms. In particular, these false alarms tend to occur in outdoor perimeter type systems, which have been installed for a period of time, upon the onset of high moisture conditions, for example, the beginning of a rain storm. It has been discovered that these false alarms are caused by resistive paths to ground formed by dirt and/or moisture on the insulators which connect the field wire to the fence posts. These resistive paths effectively reduce the impedance of the field wire as seen by the field voltage amplifier, causing its output voltage to change. This change in the field voltage, and hence in the level of the quasi-stationary electric field, is detected by the fence wire and processed as an alarm.